Mine is a Johnny Stewart Preymaster Digital Caller with a memory card collection, which includes predator calls as well as hog calls. The hog calls have four different settings - feeding hogs, distressed piglets, pigs in conflict - you get the picture. By setting your speaker near a corn pile or a heavily travelled trail, you may stir a porker's curiosity enough to draw him into the open.
I was on a Lowcountry hunt with a dozen other folks, late afternoon, and I was about a hundred yards from a corn pile. After settling into a ladder stand, I cranked up the "feeding hogs" call. After 30 minutes, I stopped. Five minutes later, I cranked up "distressed piglets." I took a break then mixed the two together - what a racket! But shortly thereafter, a 100-pound boar walked out and stood broadside. I shot. He fell. I once again cranked up the call and, right at dark, a larger sow stepped out. I shot. She fell. Not a bad afternoon's work.
What adds credibility to the Johnny Stewart call's effectiveness is that no other hunter saw a pig that evening. Everyone wanted a look at my caller. I believe Johnny Stewart's sales saw a slight spike the next day.